In many parts of the world including the U.S. there are enormous coal reserves that are becoming increasingly more important in connection with the generation of electric power. In one method of using coal reserves, coal is mixed with residual oil and fired in suspension in a refractory-lined chamber. Ceramic heat exchangers are then used to remove heat from the combustion gases laden with particles that flow past a surface of the heat exchangers. Often the heat exchange surfaces in the gas flow path are narrow in the range of from one to eight inches.
It is well known in the art that particle laden gas flow over heat exchange surfaces can cause unwanted accumulation on the surfaces which can restrict the flow of gas through a heat exchanger, and/or in less severe cases, can interfere with the rate at which thermal energy can be transferred through the heat exchange surface. Such unwanted accumulations can greatly affect the efficiency of heat exchange elements and have been a problem in the past. It has been well recognized in the industry that the deposition rate of coal ash on the heat transfer surfaces, i.e., tubes and steam generators and the like is to a large extent the function of the ash fusion point, the temperature of the gas entering the heat exchanger and the temperature and type of surface of the tubes. To some extent ash deposits can be a function of the physical configuration of the ash particles, tube spacing, gas velocities and tube arrangement in heat transfer devices. Ash could greatly affect the performance of whole systems. If such deposits could be prevented or lessened, there could be significant increases in efficiency in power generation. For example, externally fired gas turbines become much more efficient if ash accumulation can be prevented. Such systems are known and incorporate a heat exchanger combined with a suitable combustor to replace the internal combustion systems within a conventional open-cycle gas turbine. The heat exchanger accepts air at pressure at the compressor discharge conditions and heats this compressed air to a slightly higher temperature than the required turbine inlet temperature, then returns the air to the turbine section. Clean air delivered to the turbine replaces the products of combustion in the conventional internally-fired cycle thereby appreciably extending the service life of the turbine section. Static ceramic tubes are preferred for use in the heat exchangers exposed to high temperature corrosive effects of the impurities in the fuel. Designed packages for turbine cycles in the range of from 5 kilowatts to 1,350 megawatts are possible.